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Endocrine Glands
∗
OpenStax College
This work is produced by OpenStax-CNX and licensed under the
Creative Commons Attribution License 3.0†
Abstract
By the end of this section, you will be able to:
• Describe the role of dierent glands in the endocrine system
• Explain how the dierent glands work together to maintain homeostasis
Both the endocrine and nervous systems use chemical signals to communicate and regulate the body's
physiology. The endocrine system releases hormones that act on target cells to regulate development, growth,
energy metabolism, reproduction, and many behaviors. The nervous system releases neurotransmitters or
neurohormones that regulate neurons, muscle cells, and endocrine cells. Because the neurons can regulate
the release of hormones, the nervous and endocrine systems work in a coordinated manner to regulate the
body's physiology.
1 Hypothalamic-Pituitary Axis
The
hypothalamus
in vertebrates integrates the endocrine and nervous systems.
an endocrine organ located in the diencephalon of the brain.
The hypothalamus is
It receives input from the body and other
brain areas and initiates endocrine responses to environmental changes.
The hypothalamus acts as an
endocrine organ, synthesizing hormones and transporting them along axons to the posterior pituitary gland.
It synthesizes and secretes regulatory hormones that control the endocrine cells in the anterior pituitary
gland. The hypothalamus contains autonomic centers that control endocrine cells in the adrenal medulla via
neuronal control.
The
pituitary gland, sometimes called the hypophysis or master gland
is located at the base of the
brain in the sella turcica, a groove of the sphenoid bone of the skull, illustrated in Figure 1. It is attached
to the hypothalamus via a stalk called the
pituitary stalk
(or infundibulum).
The anterior portion of
the pituitary gland is regulated by releasing or release-inhibiting hormones produced by the hypothalamus,
and the posterior pituitary receives signals via neurosecretory cells to release hormones produced by the
hypothalamus. The pituitary has two distinct regionsthe anterior pituitary and the posterior pituitary
which between them secrete nine dierent peptide or protein hormones. The posterior lobe of the pituitary
gland contains axons of the hypothalamic neurons.
∗ Version
1.6: May 23, 2013 9:45 am -0500
† http://creativecommons.org/licenses/by/3.0/
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Figure 1: The pituitary gland is located at (a) the base of the brain and (b) connected to the hypothalamus by the pituitary stalk. (credit a: modication of work by NCI; credit b: modication of work by
Gray's Anatomy)
1.1 Anterior Pituitary
The anterior pituitary gland, or adenohypophysis, is surrounded by a capillary network that extends from
the hypothalamus, down along the infundibulum, and to the anterior pituitary. This capillary network is
a part of the
hypophyseal portal system that carries substances from the hypothalamus to the anterior
pituitary and hormones from the anterior pituitary into the circulatory system. A portal system carries blood
from one capillary network to another; therefore, the hypophyseal portal system allows hormones produced
by the hypothalamus to be carried directly to the anterior pituitary without rst entering the circulatory
system.
The anterior pituitary produces seven hormones:
growth hormone (GH), prolactin (PRL), thyroid-
stimulating hormone (TSH), melanin-stimulating hormone (MSH), adrenocorticotropic hormone (ACTH),
follicle-stimulating hormone (FSH), and luteinizing hormone (LH). Anterior pituitary hormones are sometimes referred to as tropic hormones, because they control the functioning of other organs.
While these
hormones are produced by the anterior pituitary, their production is controlled by regulatory hormones produced by the hypothalamus. These regulatory hormones can be releasing hormones or inhibiting hormones,
causing more or less of the anterior pituitary hormones to be secreted. These travel from the hypothalamus
through the hypophyseal portal system to the anterior pituitary where they exert their eect. Negative feedback then regulates how much of these regulatory hormones are released and how much anterior pituitary
hormone is secreted.
1.2 Posterior Pituitary
The posterior pituitary is signicantly dierent in structure from the anterior pituitary.
It is a part of the
brain, extending down from the hypothalamus, and contains mostly nerve bers and neuroglial cells, which
support axons that extend from the hypothalamus to the posterior pituitary. The posterior pituitary and
the infundibulum together are referred to as the neurohypophysis.
The hormones antidiuretic hormone (ADH), also known as vasopressin, and oxytocin are produced by
neurons in the hypothalamus and transported within these axons along the infundibulum to the posterior
pituitary. They are released into the circulatory system via neural signaling from the hypothalamus. These
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hormones are considered to be posterior pituitary hormones, even though they are produced by the hypothalamus, because that is where they are released into the circulatory system.
The posterior pituitary itself
does not produce hormones, but instead stores hormones produced by the hypothalamus and releases them
into the blood stream.
2 Thyroid Gland
The
thyroid gland is located in the neck, just below the larynx and in front of the trachea, as shown in
isthmus. It has a dark
Figure 2. It is a buttery-shaped gland with two lobes that are connected by the
red color due to its extensive vascular system. When the thyroid swells due to dysfunction, it can be felt
under the skin of the neck.
Figure 2: This illustration shows the location of the thyroid gland.
The thyroid gland is made up of many spherical thyroid follicles, which are lined with a simple cuboidal
epithelium. These follicles contain a viscous uid, called
colloid, which stores the glycoprotein thyroglobulin,
the precursor to the thyroid hormones. The follicles produce hormones that can be stored in the colloid or
released into the surrounding capillary network for transport to the rest of the body via the circulatory
system.
4 because it contains four
Thyroid follicle cells synthesize the hormone thyroxine, which is also known as T
atoms of iodine, and triiodothyronine, also known as T
cells are stimulated to release stored T
3
because it contains three atoms of iodine. Follicle
3 and T4 by thyroid stimulating hormone (TSH), which is produced
by the anterior pituitary. These thyroid hormones increase the rates of mitochondrial ATP production.
A third hormone, calcitonin, is produced by
parafollicular cells of the thyroid either releasing hormones
or inhibiting hormones. Calcitonin release is not controlled by TSH, but instead is released when calcium
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ion concentrations in the blood rise. Calcitonin functions to help regulate calcium concentrations in body
uids. It acts in the bones to inhibit osteoclast activity and in the kidneys to stimulate excretion of calcium.
The combination of these two events lowers body uid levels of calcium.
3 Parathyroid Glands
Most people have four
parathyroid glands; however, the number can vary from two to six.
These glands
are located on the posterior surface of the thyroid gland, as shown in Figure 3. Normally, there is a superior
gland and an inferior gland associated with each of the thyroid's two lobes. Each parathyroid gland is covered
by connective tissue and contains many secretory cells that are associated with a capillary network.
Figure 3: The parathyroid glands are located on the posterior of the thyroid gland. (credit: modication
of work by NCI)
The parathyroid glands produce parathyroid hormone (PTH). PTH increases blood calcium concentrations when calcium ion levels fall below normal. PTH (1) enhances reabsorption of Ca
2+ by the kidneys, (2)
stimulates osteoclast activity and inhibits osteoblast activity, and (3) it stimulates synthesis and secretion
2+
of calcitriol by the kidneys, which enhances Ca
absorption by the digestive system. PTH is produced by
chief cells of the parathyroid. PTH and calcitonin work in opposition to one another to maintain homeostatic
Ca
2+ levels in body uids.
Another type of cells, oxyphil cells, exist in the parathyroid but their function is
not known. These hormones encourage bone growth, muscle mass, and blood cell formation in children and
women.
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4 Adrenal Glands
The
adrenal glands are associated with the kidneys; one gland is located on top of each kidney as illustrated
in Figure 4. The adrenal glands consist of an outer adrenal cortex and an inner adrenal medulla. These
regions secrete dierent hormones.
Figure 4: The location of the adrenal glands on top of the kidneys is shown. (credit: modication of
work by NCI)
4.1 Adrenal Cortex
The adrenal cortex is made up of layers of epithelial cells and associated capillary networks.
form three distinct regions:
These layers
an outer zona glomerulosa that produces mineralocorticoids, a middle zona
fasciculata that produces glucocorticoids, and an inner zona reticularis that produces androgens.
+ ions in urine, sweat,
The main mineralocorticoid is aldosterone, which regulates the concentration of Na
pancreas, and saliva.
Aldosterone release from the adrenal cortex is stimulated by a decrease in blood
concentrations of sodium ions, blood volume, or blood pressure, or by an increase in blood potassium levels.
The three main glucocorticoids are cortisol, corticosterone, and cortisone. The glucocorticoids stimulate
the synthesis of glucose and gluconeogenesis (converting a non-carbohydrate to glucose) by liver cells and
they promote the release of fatty acids from adipose tissue. These hormones increase blood glucose levels to
maintain levels within a normal range between meals. These hormones are secreted in response to ACTH
and levels are regulated by negative feedback.
Androgens are sex hormones that promote masculinity.
They are produced in small amounts by the
adrenal cortex in both males and females. They do not aect sexual characteristics and may supplement sex
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hormones released from the gonads.
4.2 Adrenal Medulla
The adrenal medulla contains large, irregularly shaped cells that are closely associated with blood vessels.
These cells are innervated by preganglionic autonomic nerve bers from the central nervous system.
The adrenal medulla contains two types of secretory cells: one that produces epinephrine (adrenaline)
and another that produces norepinephrine (noradrenaline).
Epinephrine is the primary adrenal medulla
hormone accounting for 75 to 80 percent of its secretions. Epinephrine and norepinephrine increase heart rate,
breathing rate, cardiac muscle contractions, blood pressure, and blood glucose levels. They also accelerate
the breakdown of glucose in skeletal muscles and stored fats in adipose tissue.
The release of epinephrine and norepinephrine is stimulated by neural impulses from the sympathetic
nervous system. Secretion of these hormones is stimulated by acetylcholine release from preganglionic sympathetic bers innervating the adrenal medulla. These neural impulses originate from the hypothalamus in
response to stress to prepare the body for the ght-or-ight response.
5 Pancreas
The
pancreas, illustrated in Figure 5, is an elongated organ that is located between the stomach and the
proximal portion of the small intestine. It contains both exocrine cells that excrete digestive enzymes and
endocrine cells that release hormones. It is sometimes referred to as a heterocrine gland because it has both
endocrine and exocrine functions.
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Figure 5: The pancreas is found underneath the stomach and points toward the spleen. (credit:
modication of work by NCI)
The endocrine cells of the pancreas form clusters called pancreatic islets or the
islets of Langerhans,
alpha
as visible in the micrograph shown in Figure 6. The pancreatic islets contain two primary cell types:
cells,
which produce the hormone glucagon, and
beta cells,
which produce the hormone insulin.
These
hormones regulate blood glucose levels. As blood glucose levels decline, alpha cells release glucagon to raise
the blood glucose levels by increasing rates of glycogen breakdown and glucose release by the liver. When
blood glucose levels rise, such as after a meal, beta cells release insulin to lower blood glucose levels by
increasing the rate of glucose uptake in most body cells, and by increasing glycogen synthesis in skeletal
muscles and the liver. Together, glucagon and insulin regulate blood glucose levels.
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Figure 6: The islets of Langerhans are clusters of endocrine cells found in the pancreas; they stain
lighter than surrounding cells. (credit: modication of work by Muhammad T. Tabiin, Christopher P.
White, Grant Morahan, and Bernard E. Tuch; scale-bar data from Matt Russell)
6 Pineal Gland
The pineal gland produces melatonin. The rate of melatonin production is aected by the photoperiod. Collaterals from the visual pathways innervate the pineal gland. During the day photoperiod, little melatonin is
produced; however, melatonin production increases during the dark photoperiod (night). In some mammals,
melatonin has an inhibitory aect on reproductive functions by decreasing production and maturation of
sperm, oocytes, and reproductive organs. Melatonin is an eective antioxidant, protecting the CNS from
free radicals such as nitric oxide and hydrogen peroxide. Lastly, melatonin is involved in biological rhythms,
particularly circadian rhythms such as the sleep-wake cycle and eating habits.
7 Gonads
The gonadsthe male testes and female ovariesproduce steroid hormones. The testes produce androgens,
testosterone being the most prominent, which allow for the development of secondary sex characteristics and
the production of sperm cells. The ovaries produce estradiol and progesterone, which cause secondary sex
characteristics and prepare the body for childbirth.
Endocrine Glands and their Associated Hormones
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Endocrine Gland
Hypothalamus
9
Associated Hormones
releasing
and
inhibiting
Eect
hor-
mones
regulate hormone release from pituitary gland; produce oxytocin;
produce uterine contractions and
milk secretion in females
antidiuretic hormone (ADH)
water
reabsorption
from
kid-
neys; vasoconstriction to increase
blood pressure
growth hormone (GH)
promotes
growth
of
body
tis-
sues, protein synthesis; metabolic
functions
Pituitary (Anterior)
prolactin (PRL)
thyroid
stimulating
promotes milk production
hormone
(TSH)
adrenocorticotropic
lease
hormone
(ACTH)
follicle-stimulating
stimulates thyroid hormone re-
stimulates
hormone
release
by
adrenal cortex, glucocorticoids
hormone
(FSH)
stimulates
gamete
production
(both ova and sperm); secretion
of estradiol
luteinizing hormone (LH)
stimulates androgen production
by gonads; ovulation, secretion of
progesterone
melanocyte-stimulating hormone
stimulates
(MSH)
skin increasing melanin pigment
melanocytes
of
the
production.
Pituitary (Posterior)
antidiuretic hormone (ADH)
stimulates water reabsorption by
kidneys
oxytocin
stimulates
uterine
contractions
during childbirth; milk ejection;
stimulates ductus deferens and
prostate gland contraction during
emission
continued on next page
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thyroxine, triiodothyronine
Thyroid
stimulate
and
maintain
metabolism;
growth
and
de-
velopment
2+ levels
2+ levels
increases blood Ca
+ levels;
increases blood Na
+
crease K secretion
calcitonin
Parathyroid
reduces blood Ca
parathyroid hormone (PTH)
aldosterone
Adrenal (Cortex)
cortisol, corticosterone, cortisone
increase
blood
glucose
in-
levels;
anti-inammatory eects
Adrenal (Medulla)
epinephrine, norepinephrine
stimulate ght-or-ight response;
increase blood gluclose levels; increase metabolic activities
Pancreas
Pineal gland
insulin
reduces blood glucose levels
glucagon
increases blood glucose levels
melatonin
regulates
rhythms
some
and
biological
protects
CNS
from free radicals
Testes
androgens
regulate,
promote,
increase
or
maintain sperm production; male
secondary sexual characteristics
estrogen
Ovaries
promotes uterine lining growth;
female secondary sexual characteristics
progestins
promote
and
maintain
uterine
lining growth
Table 1
8 Organs with Secondary Endocrine Functions
There are several organs whose primary functions are non-endocrine but that also possess endocrine functions.
These include the heart, kidneys, intestines, thymus, gonads, and adipose tissue.
The heart possesses endocrine cells in the walls of the atria that are specialized cardiac muscle cells. These
cells release the hormone
atrial natriuretic peptide (ANP) in response to increased blood volume.
High
blood volume causes the cells to be stretched, resulting in hormone release. ANP acts on the kidneys to
+
reduce the reabsorption of Na , causing Na
+
and water to be excreted in the urine. ANP also reduces the
amounts of renin released by the kidneys and aldosterone released by the adrenal cortex, further preventing
the retention of water. In this way, ANP causes a reduction in blood volume and blood pressure, and reduces
+
the concentration of Na
in the blood.
The gastrointestinal tract produces several hormones that aid in digestion. The endocrine cells are located
in the mucosa of the GI tract throughout the stomach and small intestine. Some of the hormones produced
include gastrin, secretin, and cholecystokinin, which are secreted in the presence of food, and some of which
act on other organs such as the pancreas, gallbladder, and liver. They trigger the release of gastric juices,
which help to break down and digest food in the GI tract.
While the adrenal glands associated with the kidneys are major
endocrine glands, the kidneys them-
selves also possess endocrine function. Renin is released in response to decreased blood volume or pressure
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and is part of the renin-angiotensin-aldosterone system that leads to the release of aldosterone. Aldosterone
+ and water, raising blood volume. The kidneys also release calcitriol, which
2+
Ca
and phosphate ions. Erythropoietin (EPO) is a protein hormone that
then causes the retention of Na
aids in the absorption of
triggers the formation of red blood cells in the bone marrow. EPO is released in response to low oxygen
levels. Because red blood cells are oxygen carriers, increased production results in greater oxygen delivery
throughout the body. EPO has been used by athletes to improve performance, as greater oxygen delivery to
muscle cells allows for greater endurance. Because red blood cells increase the viscosity of blood, articially
high levels of EPO can cause severe health risks.
thymus
The
is found behind the sternum; it is most prominent in infants, becoming smaller in size
through adulthood.
The thymus produces hormones referred to as thymosins, which contribute to the
development of the immune response.
Adipose tissue is a connective tissue found throughout the body.
It produces the hormone
leptin
in
response to food intake. Leptin increases the activity of anorexigenic neurons and decreases that of orexigenic
neurons, producing a feeling of satiety after eating, thus aecting appetite and reducing the urge for further
eating. Leptin is also associated with reproduction. It must be present for GnRH and gonadotropin synthesis
to occur. Extremely thin females may enter puberty late; however, if adipose levels increase, more leptin
will be produced, improving fertility.
9 Section Summary
The pituitary gland is located at the base of the brain and is attached to the hypothalamus by the infundibulum. The anterior pituitary receives products from the hypothalamus by the hypophyseal portal system and
produces six hormones. The posterior pituitary is an extension of the brain and releases hormones (antidiuretic hormone and oxytocin) produced by the hypothalamus.
The thyroid gland is located in the neck and is composed of two lobes connected by the isthmus. The
thyroid is made up of follicle cells that produce the hormones thyroxine and triiodothyronine. Parafollicular
cells of the thyroid produce calcitonin. The parathyroid glands lie on the posterior surface of the thyroid
gland and produce parathyroid hormone.
The adrenal glands are located on top of the kidneys and consist of the renal cortex and renal medulla.
The adrenal cortex is the outer part of the adrenal gland and produces the corticosteroids, glucocorticoids,
and mineralocorticoids.
The adrenal medulla is the inner part of the adrenal gland and produces the
catecholamines epinephrine and norepinephrine.
The pancreas lies in the abdomen between the stomach and the small intestine. Clusters of endocrine
cells in the pancreas form the islets of Langerhans, which are composed of alpha cells that release glucagon
and beta cells that release insulin.
Some organs possess endocrine activity as a secondary function but have another primary function. The
heart produces the hormone atrial natriuretic peptide, which functions to reduce blood volume, pressure,
+
and Na
concentration. The gastrointestinal tract produces various hormones that aid in digestion. The
kidneys produce renin, calcitriol, and erythropoietin. Adipose tissue produces leptin, which promotes satiety
signals in the brain.
10 Review Questions
Exercise 1
Which endocrine glands are associated with the kidneys?
a. thyroid glands
b. pituitary glands
c. adrenal glands
d. gonads
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Exercise 2
12
(Solution on p. 13.)
Which of the following hormones is not produced by the anterior pituitary?
a. oxytocin
b. growth hormone
c. prolactin
d. thyroid-stimulating hormone
11 Free Response
Exercise 3
(Solution on p. 13.)
What does aldosterone regulate, and how is it stimulated?
Exercise 4
(Solution on p. 13.)
The adrenal medulla contains two types of secretory cells, what are they and what are their
functions?
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Solutions to Exercises in this Module
to Exercise (p. 11)
C
to Exercise (p. 11)
A
to Exercise (p. 12)
The main mineralocorticoid is aldosterone, which regulates the concentration of ions in urine, sweat, and
saliva. Aldosterone release from the adrenal cortex is stimulated by a decrease in blood concentrations of
sodium ions, blood volume, or blood pressure, or an increase in blood potassium levels.
to Exercise (p. 12)
The adrenal medulla contains two types of secretory cells, one that produces epinephrine (adrenaline) and
another that produces norepinephrine (noradrenaline). Epinephrine is the primary adrenal medulla hormone
accounting for 7580 percent of its secretions. Epinephrine and norepinephrine increase heart rate, breathing
rate, cardiac muscle contractions, and blood glucose levels. They also accelerate the breakdown of glucose
in skeletal muscles and stored fats in adipose tissue.
The release of epinephrine and norepinephrine is
stimulated by neural impulses from the sympathetic nervous system. These neural impulses originate from
the hypothalamus in response to stress to prepare the body for the ght-or-ight response.
Glossary
Denition 1: adrenal cortex
outer portion of adrenal glands that produces corticosteroids
Denition 2: adrenal gland
endocrine glands associated with the kidneys
Denition 3: adrenal medulla
inner portion of adrenal glands that produces epinephrine and norepinephrine
Denition 4: alpha cell
endocrine cell of the pancreatic islets that produces the hormone glucagon
Denition 5: anterior pituitary
portion of the pituitary gland that produces six hormones; also called adenohypophysis
Denition 6: atrial natriuretic peptide (ANP)
hormone produced by the heart to reduce blood volume, pressure, and Na
Denition 7: beta cell
+
concentration
endocrine cell of the pancreatic islets that produces the hormone insulin
Denition 8: colloid
uid inside the thyroid gland that contains the glycoprotein thyroglobulin
Denition 9: endocrine gland
gland that secretes hormones into the surrounding interstitial uid, which then diuse into blood
and are carried to various organs and tissues within the body
Denition 10: erythropoietin (EPO)
hormone produced by the kidneys to stimulate red blood cell production in the bone marrow
Denition 11: hypophyseal portal system
system of blood vessels that carries hormones from the hypothalamus to the anterior pituitary
Denition 12: islets of Langerhans (pancreatic islets)
endocrine cells of the pancreas
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Denition 13: isthmus
tissue mass that connects the two lobes of the thyroid gland
Denition 14: leptin
hormone produced by adipose tissue that promotes feelings of satiety and reduces hunger
Denition 15: pancreas
organ located between the stomach and the small intestine that contains exocrine and endocrine
cells
Denition 16: parafollicular cell
thyroid cell that produces the hormone calcitonin
Denition 17: parathyroid gland
gland located on the surface of the thyroid that produces parathyroid hormone
Denition 18: pituitary gland
endocrine gland located at the base of the brain composed of an anterior and posterior region; also
called hypophysis
Denition 19: pituitary stalk
(also, infundibulum) stalk that connects the pituitary gland to the hypothalamus
Denition 20: posterior pituitary
extension of the brain that releases hormones produced by the hypothalamus; along with the
infundibulum, it is also referred to as the neurohypophysis
Denition 21: thymus
gland located behind the sternum that produces thymosin hormones that contribute to the development of the immune system
Denition 22: thyroid gland
endocrine gland located in the neck that produces thyroid hormones thyroxine and triiodothyronine
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